Abstract: Mortality from infectious diseases represents the second leading cause of death worldwide, making the development of new vaccines an important priority of biomedical research. Traditional vaccination strategies are very effective at generating neutralizing antibodies against bacteria and viruses. However, a vaccine capable of generating durable T cell immunity is still beyond our reach, due, in part, to the uncertainty of how effector and memory T cell subsets arise from their naive predecessors. A major challenge in the field, thus, is to understand how this cellular diversity is achieved during an immune reponse. Recently we have provided evidence that this divergence in cell fate may occur as early as the first T cell division of an immune response, using an evolutionarily conserved strategy for diversification known as asymmetric division. However, the regulation and functional consequences of asymmetric T cell division during immune responses have not been adequately studied. This is related to the inability to reversibly disrupt potential regulators of asymmetry selectively during the first T cell division of an immune response, when asymmetric division has been shown to occur. This proposal focuses on: (1) developing a new method to allow reversible inhibition of gene targets in naive T cells in vivo and (2) using a multi- disciplinary approach to investigate major mechanisms underlying asymmetric T cell division during immune responses. Ultimately, these studies could suggest modulation of asymmetric T lymphocyte division as a novel strategy to maximize memory T cell responses during a microbial challenge. Public Health Relevance: The purpose of vaccination is to induce a long-lived immune response against a microbe. Memory T lymphocytes are cells of the immune system that provide longlived protection against microbes. This proposal seeks to understand how memory T lymphocytes are generated.